The act of walking out of cover, stopping, slowly turning and killing your opponent in 1 shot

then in all caps typing "MERRY CHRISTMAS"

this is not just a seasonal greeting, it can be used all throughout the year

you are sitting watching your brother play an fps, a guy named ironman545 walks out from behind cover, turns 90 degrees very slowly and then shoots him in the head
your brother never fired a shot and you also had no idea what to do in such a situation
you both turn to eachother and say, that guy just santa claused us